Well it should be a SAE 10W, 30 or 50 that meets the TDTO (Transmission Drive Train Oil) oil spec from Caterpillar.
Dependent on your customers application will depend on which viscosity of oil you'll use, as I'm sure you well aware of.
The oil will ressemble engine oil in colour and general viscosity but if the right oil is used its properties are slightly different. I would suggest you contact a local Caterpillar dealer to get the right oil.
When I used to work on these units we did use engine oil and hydraulic oil in instances where we could not obtain SAE 30 TDTO oil (As it is only supplied these days in the UK in drums), we would use SAE 10 hydraulic oil, especially if the 30 that was in it was causing it to have sluggish pick up on cold mornings. However engine and hyd oils have more 'slippage' (For want of a better word) properties as part of their design, so potentially it could cause more clutch slippage......so would not recommend it these days.
If you can't get the Cat oil then use one that meets API CD/TO-2 or military specification MIL-L-2104D.
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